In the field of medicine and biological sciences, it is desirable to perform qualitative analysis of certain substances with speed and accuracy. When the subject specimen is a liquid, such as, blood or urine, examination is often undertaken by isolating and fixing a small amount of the substance on a transparent plate for viewing through a lighted optical instrument, such as, a microscope. This procedure is especially suited for the observation and evaluation of particulate matter in the specimen; for instance, estimation of sediment in urine or enumeration of red blood cells. Accordingly, it is important that the liquid be spread into a very thin layer of uniform depth.
An early approach to "wet mount" examination of specimens, still very much in use today, involves depositing a drop of the liquid onto a smooth, transparent plate of glass or plastic. A thin cover slip is then placed over the plate to protect and retain the specimen and distribute it as a fairly thin, even film over the viewing area. A disadvantage of this practice is the tendency to introduce too much liquid onto the slide surface, resulting in overflow off the slide surface, displacement of the cover slide, and an uneven or undesirably thick layer of the specimen over the viewing area. Moreover, preparation and handling of the flat slides and fragile cover slips is unnecessarily awkward, often resulting in spillage or runoff and possible contamination of the liquid sample. Finally, each slide can accommodate only one specimen at a time; if comparison between samples is desired, then slides must be prepared and viewed separately, which is time-consuming and necessitates even further handling.
Previous attempts to overcome the foregoing limitations have generally proposed that the slide be provided with an arrangement of channels in communication with a somewhat raised viewing surface. A cover slip is positioned over the viewing area so that liquid placed in the channel will be drawn across the space by the process of capillary action. Representative of this approach is U.S. Pat. No. 2,039,219 to Hausser et al which discloses a haemacytometer having a series of transverse channels and a pair of oppositely inclined surfaces leading to a raised examination area. A cover slide is placed over the raised and channeled areas and a narrow chamber is formed therebetween. The specimen is introduced, as by a dropper, onto one of the inclined portions and is drawn into the chamber; excess liquid remains in the reservoirs formed by the inclined surfaces.
U.S. Pat. No. 3,565,537 to Fielding teaches a somewhat similar approach to a blood specimen holder for use with an optical measuring instrument. This arrangement features a pair of translucent plastic plates, one being formed with a shallow recess, which are welded together to form an intermediate channel for receiving the liquid sample by capillary action. The slide may then be examined under an optical instrument specifically designed to measure the color of blood and thereby determine hemoglobin content.
A further example of the art in this field is U.S. Pat. No. 3,777,283 to Elkins which defines a unitary plastic slide adapted to accommodate a plurality of specimens. Each sample is retained by capillary action in an open-sided chamber which is molded to a predetermined area and depth within the body of the slide. Excess liquid remaining in an introduction channel leading to the chamber may be removed by tapping the open or channel side of the slide against absorbent toweling or the like. Further, as the specimen chamber is formed integrally with the slide, this arrangement does not employ a cover slip.